Well, it’s a somewhat complex topic, as genders in Pali are not as simple as “make the last vowel long and it’s feminine”. If that was the case, what would we say of the Buddha, constantly referred to as bhagavā? This kind of usage is not at all unusual. See, for example, the following list of epithets for a “wise person” I just stumbled across, almost all with long final vowels, and all masculine singular:
Paññāṇavā soti paṇḍito paññavā buddhimā ñāṇī vibhāvī medhāvī
One complication is that names in Pali are typically quoted in the nominative form, whereas in foreign languages they are usually used in stem (= vocative) form. But this is not always the case; sujāto is nominative singular masculine. This is an interesting case, as in “standard” form we’d present my name as “Sujata”, and indeed I tried to use this for a while. But everyone thought it sounded like a girl’s name! I didn’t mind, but hey, who am I to question the will of the majority?
Sometimes, as mentioned by several commenters, a name may simply “sound” right even if it’s not correct in Pali. For example, satimā is used as a feminine name meaning “mindful one”, whereas in fact it is a masculine form. The feminine is satimantī.
Somewhat of a digression here, but back to the topic. Pasanna should indeed be declined in a feminine form, pasannā. But anyway, most of us, including myself, never worry about using diacriticals for names, so it’s really just a matter of preference.
As for vimalañāṇi, the final i here represents the in ending, which can in theory be either long or short in both masculine and feminine forms. However, as it happens I can only find occurrences in the canon in a masculine form using the long ī. So regardless of gender it seems the long ī is the idiomatic form.
One little detail in this that I believe has not been noticed before. Pali has a third gender, the neuter. This is used for things that are neither male nor female, such as the mind (citta). (Of course it’s not as simple as that, as lots of ungendered things are linguistically gendered anyway, usually to masculine by default.)
While the neuter gender hasn’t traditionally been used for names, it could be used by people who don’t want to adopt a binary gender. In this case, the neuter form is usually declined—in both nominative and (the extremely rare) vocative—with a closing ṁ. So pasannaṁ, sujātaṁ, vimalañāniṁ.